On Mon, May 16, 2011 at 8:10 AM, Robert Clausecker
I found out, that GHC implements typeclasses as an extra argument, a record that stores all functions of the typeclass. So I was wondering, is there a way (apart from using newtype) to pass a custom record as the typeclass record, to modify the behavior of the typeclass? I thought about something like this:
Would GHC's implicit parameter extension possibly suit your purposes here? Your example would translate as: {-# LANGUAGE ImplicitParams #-} type ShowClass a = a -> String f :: (?showC :: ShowClass a) => [a] -> String f x = x >>= ?showC g :: [Int] -> String g = let ?showC = show in f g2 :: [Int] -> String g2 = let ?showC = (return . toEnum) in f ...where: > g [72, 97, 115, 107, 101, 108, 108] "7297115107101108108" > g2 [72, 97, 115, 107, 101, 108, 108] "Haskell" Clearly this doesn't allow you retrofit such functionality onto existing code using existing type classes, but I'd be wary of doing that anyway--type class instances are not something that code will expect to have changing out from under it. Otherwise, this seems to be exactly what the implicit parameters extension is designed for, judging from the way the GHC user's guide describes it. - C.